If you receive the following error message from your QNAP NAS:

The file system is not clean. It is suggested that you run „check disk“

and a disk check is failed with that message:

[Mirror Disk Volume: Drive 2 3 4 1] Examination failed (Cannot unmount disk).

ssh your qnap and do the following:

/etc/init.d/services.sh stop && /etc/init.d/opentftp.sh stop && /etc/init.d/Qthttpd.sh stop
umount /dev/md0
e2fsck -f -v -C 0 /dev/md0
mount /dev/md0
reboot

If you receive the following error while trying to unmount the device:
umount: /share/MD0_DATA: device is busy

try to figure out which process is still using the device:

# lsof +f -- /dev/md0
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
sh 10721 admin cwd DIR 9,0 4096 16252936 /share/MD0_DATA/
lsof 29119 admin cwd DIR 9,0 4096 16252936 /share/MD0_DATA/
lsof 29120 admin cwd DIR 9,0 4096 16252936 /share/MD0_DATA/

Which indicates that your current shell is trying to use the device, simply use: cd / and umount again.

That should help